Materials that convert x-ray energy into visible light are useful for detection and imaging purposes. Materials that are exceptionally efficient at this conversion or that give higher resolution images and exhibit reduced afterglow are preferred. The glass material of the present invention is specifically intended for converting x-ray electromagnetic energy into visible electromagnetic energy for the purpose of detecting and imaging these radiations.
A method for using luminescent glass is to view the emission in real time employing a video camera. Important characteristics of the glass in this mode include high absorption of the excitation radiation, high luminescence responsivity, prompt emission, low afterglow, and a linear response over a wide range of excitation intensity. The luminescent glass must also emit light within the wavelength sensitivity range of the recording camera and maintain the spatial resolution of the primary image. Particular applications for the glass in this mode include detecting x-rays, or gamma rays for the purpose of diagnostic medical or industrial imaging (see, for example, Bossi, R. H. , Oien, C. T., "Real-Time Radiography" RPT #DE82-005603, UCRL-53091, LLNL, CA) or for tracking high energy particles (see, for example, Ruchti, R. et al, "Scintillating Glass, Fiber Optic Plate Imaging System for Tracking Applications in High Energy Physics Experiments", J. Luminescence Vol. 31-32, Part 1 and 2, December 1984, Proceedings of the 1984 International Conference on Luminescence, Madison, Wis., USA, Aug. 13-17, 1984 p. 721-723 ).
Luminescent glasses that are transmissive to light from the near ultraviolet (200-300 nm) to the near infrared (3000 nm) and that can be formed into clear glass plates and fibers (see R. F. Reade, U.S. Pat. No. 3,654,172), lenses (see T. Takahashi, et al, U.S. Pat. No. 4,259,587), or ground glass are used to detect various forms of electromagnetic, electron beam, particle beam or nuclear radiation by luminescing under the proper excitation conditions. The threshold for luminescence in glass as in other inorganic luminescent materials is typically above an excitation energy of approximately 2.5 eV (see, for example, Blasse, G. and Bril, A. "Characteristic Luminescence," Philips Technical Review, Vol. 31, No. 10, 304, 1970).
Luminescent glasses are of particular interest in real time x-ray radiography. Traditionally, in real time x-ray radiography, a visible image is generated by a polycrystalline x-ray-to-light conversion screen that is monitored by a low light level TV camera. A similar screen can also be employed in an x-ray image amplifier tube where the visible image generated by the phosphor is further electro-optically intensified prior to collection by a TV camera. Other types of x-ray screens are typically composed of polycrystalline x-ray phosphor particles embedded in a binder, and both are supported on a thin mylar sheet as taught by Buchanan, R. A. et al, U.S. Pat. No. 3,725,704. Although many commercial polycrystalline phosphor screens have a strong luminescence signal under x-rays (as illustrated by Buchanan, R. A., et al in U.S. Pat. No. 4,297,584 ) , the image resolution is limited by the light scatter among individual phosphor particles in the screen and the image contrast can be limited by their relatively low x-ray absorption efficiencies. Furthermore, light scatter in thicker, more absorbing screens will trap the light embedded in the sublayers of the screen to result in a reduced light signal and therefore a reduced contrast perceptibility. The binder in the screen, which can be present in as much as 50 percent by weight has the added disadvantage of being inert and provides no means for transferring energy absorbed within to luminescent centers in the crystalline phosphor particles. These same problems exist when these phosphor screens are used as film intensifier screens in radiographic film cassettes.
Luminescent glass plates and fiber optic scintillating plates offer solutions to the problems imposed by polycrystalline phosphor screens. Such glass plates do not degrade resolution by transverse light scattering within the glass. Secondly, increased x-ray absorption and improved noise statistics can be realized by the use of thicker (1/4") glass plates where light from the entire cross-section of the plate can reach the recording medium without being trapped or scattered. Thirdly, no binder is required, and theoretically all the x-ray energy absorbed can be transferred to the luminescent centers in the glass. Finally, luminescent glass plates are more resistant to environmental attack, for example, scratches, chemical reaction, and abrasion.
The advantage of using thick glass plates is particularly important for industrial radiography where high energy x-rays (100keV-16MeV) are used and where the thick glass plates will absorb and convert a higher portion of the photon flux than the more x-ray transparent thin polycrystalline phosphor screens. Illustrative uses for high energy luminescent glasses include a stand alone luminescent glass plate viewed by a low level TV camera forming a real time radiographic system, a fiber optic scintillating plate incorporated in an x-ray image amplifying fluoroscopy tube, a fiber optic scintillating plate for intensification of film in a radiographic film cassette and a fiber optic scintillating plate in direct contact with a CCD in a CCD camera in a solid state x-ray imaging system.
Although luminescent glass detection screens have shown promise for x-ray radiographic application (U.S. Pat. No. 3,654,172), they haven't been widely used because of phosphorescence problems including signal instability (i.e., background buildup), afterglow and low luminescence responsivity. Those that have been used are activated with terbium oxide, but suffer from these drawbacks.
The effects of phosphorescence, afterglow and background buildup are believed to be caused by radiation produced free electrons and holes that are trapped in defect centers in the glass structure. Such defect centers can consist of vacancies, interstitial atoms or other types of charge imbalance. Over time, these trapped carriers are released by thermal and electromagnetic (i.e., x-rays) stimulation, and radiatively recombine at luminescence centers to result in phosphorescence. In real time systems, the buildup of the background is a result of the gradual filling of traps. With increasing illumination, intensity or duration, deeper and deeper traps become filled, and eventually emptied and hence the long decay component is increased and the background level increases. If the background levels are significant, this could lead to increased noise and image lag. The problem becomes apparent in making observations of very weak signals following very strong signals. This problem is not limited to luminescent glasses but has also arisen with some polycrystalline phosphor materials in real time imaging systems (see, for example, Torr, M. R. "Persistence of Phosphor Glow in Microchannel Plate Image Intensifiers," Applied 5 Optics Vol. 24, No. 6, 793, 1985).
Prior terbium activated silicate luminescent glasses used in x-ray applications, including those developed under U.S. Pat. No. 3,654,172, have a strong phosphorescence after only moderate exposures to x-rays. Applicants have found that this phosphorescence is manifested as a signal increase of as much as 130% after 3000 R and 180% after 25,000 R of its initial value under continuous illumination at 250 kVp.
The prior art also contains many examples of terbium activated silicate luminescent glass developed for use in lamps, where the luminescent material is used to transform ultraviolet (UV) electromagnetic radiation into visible electromagnetic radiation. While these materials are similar to those of the present invention in that they are both terbium activated silicate glass materials, these materials, however, have high afterglow and trapping of x-ray energy under x-ray excitation. Furthermore, these materials do not contain the high Z components needed for x-ray absorption and would need to be excessively thick to absorb the appropriate levels of radiation. X-ray absorption in materials increases by Z.sup.3 in the diagnostic x-ray energy regime. Increasing the effective Z of the glass will therefore have a substantial impact on its x-ray absorption efficiency and therefore the signal-to-noise of the resulting image. Conversely, higher Z materials will allow thinner plates to be obtained for a given x-ray absorption efficiency. Thinner x-ray screens of a given class of luminescent materials always provides improved spatial resolution. Examples of luminescent glass materials that are activated by trivalent terbium but are poor x-ray materials because of high afterglow and low x-ray absorption efficiency are those developed by (1) Popma, et al, U.S. Pat. No. 4,751,148, (2) Santoku Met Ind KK, 12.09.73-JA-102141 (09.05.75) CO3b DO3d, (3) Oversluizen, et al, U.S. Pat. Nos. 4,798,681 and 4,798,768, and (4) Barber, et al, U.S. Pat. No. 3,527,711.
These materials developed for UV lamp applications have a different composition, and have a different physical operating behavior from those materials of the present invention. The UV excitation process is understood in terms of the UV excitation energy interacting directly with the emitting activator ion, whereas in the x-ray case the excitation energy interacts with the host materials producing electrons and holes which then interact with the emitting activator ion. In the x-ray excitation process there is opportunity for the electrons and holes to become temporarily trapped in the host material before transmitting their energy to the activator ion. This trapping effect results in unwanted afterglow and persistence in the emitted radiation. This afterglow is detrimental to the x-ray uses of these materials. It is recognized in the art that a good material for use in lamps is not indicative of the material's performance in x-ray screens and one cannot depend on the known properties of UV-excited lamp materials as a guide to x-ray performance.
While the prior art teaches many compositions that are useful for UV lamps and cathode-ray tube applications, the prior art does not give any guidance for reducing phosphorescence, buildup or afterglow in these materials while retaining or improving luminescence responsivity under x-rays.
Another aspect limiting the use of existing luminescent glasses for x-ray imaging applications is their low luminescence efficiency (gram for gram) compared to polycrystalline phosphor materials. The known prior art luminescent glass host materials allow only a low level of terbium activation before the onset of concentration quenching (where increased levels of activator do not increase light output and can reduce responsivity by increasing the probability of trapping in the glass) . There exists a need for new host materials permitting increased levels of terbium oxide before the onset of concentration quenching with a resulting improvement in light output.